Heat treatment, which is commonly used in various technical fields, is a critical process for material growth, synthesis or modification. Generally, through heat treatment, elements or compounds in precursors are subjected to various physical or chemical reactions such as bond formation, diffusion, crystallization, recrystallization, dissociation and vaporization. The physical and chemical properties of products may be modified by adjusting the operational conditions in heat treatment methods such as temperature gradient, temperature holding duration, cooling rate, species of surrounding gas, gas pressure, gas flow or thermal flow. Such operational conditions in heat treatment are important and have a great influence on product properties. Particularly for commercialized products which are mass manufactured, such operational conditions are critical factors in a product's homogeneity and yield.
In order to investigate and obtain the suitable heat treatment conditions, numerous experiments and tests are needed. However, such experiments and tests are not only costly and time-consuming at the research stage but also usually require expensive equipment at the subsequent commercialization stage to control or maintain operational conditions. Especially for production related to heat treatment on large substrates such as solar panels, such issue becomes a major concern. It has become a primary objective for persons in the field of solar cells to develop a cost efficient heat treatment process that can produce products with homogeneous properties and improved yield.
Attempts were made in the prior art to improve the product yield and increase cost efficiency. For example, US 2011/0088768 A1 discloses the most preferable temperature for heat treatment for the targeted products; US 2010/0203668 A1 provides the most preferable heat profile, pressure and species of the surrounding gas for the heat treatment of metallic precursors; and TW M413213 provides an annealing chamber to optimize gas flow distribution. However, such technical means which are generally directed to the adjustment of heat treatment parameters still require a significant extent of equipment modification. Given the above, a different approach from such parameter adjustments is taken by the present invention. The present invention provides homogeneous products and achieves high stability and yield of product manufacture simply by applying a protective layer directly onto a precursor so that the precursor on each portion of the substrate is treated at substantially the same conditions.